Project Summary Allogeneic hematopoietic-cell transplantation (allo-HCT) is an important treatment for hematological malignancies. The intestinal microbiota consists of a community of diverse microbes that reside in the intestine and are critical for host development, homeostasis, and immune regulation. In human analyses and animal experiments, we and others have shown that the intestinal microbiota contribute to the pathophysiology of all three major complications of allo-HCT: infections, GVHD, and relapse. Using 16S ribosomal RNA next- generation sequencing, we examined the intestinal microbiota of allo-HCT patients and found a post-transplant ?microbiota injury?. This dysbiosis is likely due to the combined effects of (a) broad-spectrum antibiotics for the treatment of post-transplant febrile neutropenia and (b) the profound nutritional alterations experienced by these patients. We found an inverse relationship between a loss of the genus Blautia after allo-HCT and GVHD mortality. We observed that broad-spectrum antibiotics that target the anaerobic commensal flora are particularly associated with increases in GVHD-related mortality and in fact worsened intestinal GVHD in our animal model. We and others have observed an association between Enterococcus and the development of GVHD in mouse and man. Finally, we have found an association between Eubacterium limosum and graft-vs- tumor activity after allo-HCT. Therefore, we hypothesize that the intestinal microbiota can modulate alloreactivity. We propose in Aim 1 to identify microbiota biomarkers of GVHD in allo-HCT patients and to develop a clinically useful multi-parameter, rapid-turnaround biomarker panel. In Aim 2, we will use mouse models to study the mechanisms by which members of the microbiota affect alloreactivity, including both GVHD and graft-vs-tumor activity. In addition to elucidating the interactions of the intestinal microbiota in the development of GVHD, this work will form the basis of clinical trials to reduce GVHD and transplant-related mortality. During the award period, the candidate will conduct research at Memorial Sloan Kettering Cancer Center under the mentorship of Dr. Marcel van den Brink and an Advisory Committee. He will obtain the critical skills he needs to become a tenure-track physician-scientist who runs his own academic laboratory and competes for independent NIH funding. He will acquire detailed and comprehensive knowledge of mouse microbiota models, statistical methods in biomarker development, and computational-biology approaches to the analysis of high-dimensional microbiome sequencing data.